A275245 - OEIS

A275245

Numbers k such that phi(k) divides k^2 while phi(k) does not divide k.

10, 20, 40, 42, 50, 60, 80, 84, 100, 114, 120, 126, 136, 156, 160, 168, 180, 200, 220, 228, 240, 250, 252, 272, 294, 300, 312, 320, 336, 342, 360, 378, 400, 440, 444, 456, 468, 480, 500, 504, 540, 544, 588, 600, 624, 640, 672, 684, 720, 756, 800, 816

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

10 is a term because phi(10) = 4; 10 mod 4 = 2 and 10^2 mod 4 = 0.

MATHEMATICA

Select[Range[10^3], Function[k, And[Divisible[#^2, k], ! Divisible[#, k]]]@ EulerPhi@ # &] (* Michael De Vlieger, Jul 21 2016 *)

PROG

(PARI) isok(n) = (n % eulerphi(n) != 0) && (n^2 % eulerphi(n) == 0)

CROSSREFS